A lot of the issue with any form of government-paid education is, who exactly will pay for it? The overall annual expense would most likely run into the hundreds of billions, which is a lot of money but perfectly manageable if taxes rise accordingly. Now the real question is, who exactly is footing the bill: the federal, state, or local government? And more importantly, who is going to be willing to make the controversial laws that will raise taxes and make reforms that are going to be a huge headache for everyone involved for many years to come, regardless of the ultimate outcome? Probably no one for a long while.
What tends to happen is that the federal government controls what it can control without much controversy (federal aid and loan interest rates) and the state and local governments each do as they do (there are a lot of them, of different means). The result is a bit of a mess.